The processing of chemical cellulosic pulps in the manufacture of various grades of paper and paper products generally requires that such pulps be subjected to several successive bleaching treatments. These bleaching treatments are optionally interspersed with various washing, dilution, extraction and/or concentration stages in order to arrive at a final product having a desired brightness.
It has been conventional for many years to delignify and bleach wood pulp by using elemental chlorine. Environmental as well as processing problems with chlorine bleaching have led to the development of bleaching processes in which oxygen replaces chlorine as the primary pulp bleaching agent. Oxygen, however, is not as selective a delignification agent as elemental chlorine. The lignin content of the pulp can be reduced only to a limited extent before the oxygen attacks the cellulosic fibers therein. Although the remaining lignin can be removed with chlorine and/or chlorine dioxide, much research in recent years has been devoted to the search for effective ways in which oxygen-bleached pulp can be further delignified with chemicals other than chlorine compounds.
Ozone has been studied as an alternative further bleaching agent for oxygen-bleached pulp. Although ozone may initially appear to be an ideal material for bleaching lignocellulosic materials, its exceptional oxidative properties and relatively high cost have limited the development of satisfactory ozone bleaching processes, especially for the intractable southern softwoods. Ozone will readily react with lignin to effectively reduce the K Number but it will also under most conditions aggressively attack the carbohydrate of the cellulosic fibers, thus substantially reducing the strength of the resulting pulp.
In an effort to overcome these disadvantages, those working in this field have extensively examined numerous alternative bleaching processes designed to reduce or eliminate the use of elemental chlorine and chlorine-containing compounds from multi-stage bleaching processes for lignocellulosic pulps. These alternative processes utilize, for example, various combinations of oxygen ("O"), ozone ("Z"), alkaline extraction ("E") and peroxides ("P"), to name the primary chemicals used. Complicating these efforts, however, is the requirement that high levels of pulp brightness are necessary for many of the applications for which such pulp is to be used. The prior art processes which utilize these materials in various combinations are, however, often unable to achieve these high pulp brightness levels without an unacceptable loss in pulp strength.
One commercially successful chlorine-free bleaching sequence is disclosed by Griggs et al. in U.S. Pat. Nos. 5,164,043 and 5,164,044. These patents disclose multi-stage processes for delignifying and bleaching a lignocellulosic material. Initially, a pulp is formed from the lignocellulosic material by Kraft pulping, Kraft AQ pulping or extended delignification. The pulp is then partially delignified with oxygen preferably according to a modified alkaline addition technique where the alkaline material is substantially uniformly combined with the pulp at low consistency prior to removing pressate and forming a high consistency pulp which is then contacted with the oxygen. Next, the partially delignified pulp is treated with a chelating agent and an acid to a pH range of about 1 to 4, and the pulp is then further delignified with ozone. Preferably, the ozone stage is conducted on high consistency pulp utilizing a dynamic reactor which turbulently mixes the pulp with the ozone gas so that substantially all pulp particles are exposed to the ozone gas for reaction therewith. This enables the pulp to be substantially uniformly bleached, thus forming an intermediate pulp.
The pulping/oxygen/ozone process taught by Griggs et al. produces intermediate pulps having a GE brightness of at least about 50%. For most papermaking purposes, however, a GE brightness in the range of 50 to 65% is unsatisfactory. In order to raise the GE brightness further to the more desirable levels of 90% or higher, the pulp is subjected to brightening bleaching, which is primarily intended to convert the chromophoric groups on the lignin remaining in the pulp into a colorless state.
Chlorine dioxide is generally highly effective both as a pulp brightness bleaching agent as well as a delignifying agent. As taught by Griggs et al., an appropriate amount of chlorine dioxide can be used, after an alkaline extraction of pulp ozonated in accordance with the inventive process described in the patent, to prepare high-strength pulp having a GE brightness value greater than 80%. Where, extremely high pulp brightnesses of about 92% GEB are desired, Griggs et al. teaches that additional extraction and chlorine dioxide treatments would be appropriate.
The Griggs et al. patent further teaches that hydrogen peroxide may be used instead of chlorine dioxide. When utilizing peroxides as the bleaching agent, however, the K Number of the pulp should be reduced to about 6 prior to the ozonation step in order to obtain a product having a GE brightness of greater that 80% following the peroxide bleaching stage, since peroxide is not as effective a bleaching agent as chlorine dioxide.
Cael, U.S. Pat. No. 4,404,061, teaches that persulfate, conveniently in the form of Oxone (which is defined hereinbelow), may be used to bleach Northern hardwood kraft pulp at 50.degree. C., and that it may be used to pretreat Northern softwood chips prior to pulping them.
Hammann et al., "Bleaching of Kraft Pulp and ASAM Pulp without Chlorine Containing Chemicals", Preprints from the International Pulp Bleaching Conference 1991, Stockholm, Sweden, published by The Swedish Association of Pulp and Paper Engineers, 1991 (3) 185, discloses processes in which alkali-neutralized caroic acid (initial pH=10.3) is used in high temperature (70.degree. C.) bleaching sequences on Kraft and ASAM pulps. Hammann et al. teach that this high temperature alkaline caroic acid bleaching stage allows sufficient pulp brightness after final bleaching without overly affecting the technological properties (i.e., strength) of the pulp. The measures of strength referred to in Hammann et al. are "breaking length" ("BL") and "tear strength" (TS). In Table 2a, an ozonation stage is reported to reduce Kraft pulp BL by 0.1 units and TS by 5.0 units; in the same table, caroic acid (applied at 70.degree. C. and pH 10) lowers the BL and TS a further 0.05 and 2.4 units, respectively. In Table 3a, an oxygenation stage is reported to reduce Kraft pulp BL by 0.07 units and TS by 17.7 units; in the same table, caroic acid (applied at 70.degree. C. and pH 10) lowers the BL and TS a further 0.58 and 1.4 units, respectively. Finally, in Table 4, an ozonation stage is reported to reduce ASAM pulp BL by 0.53 units and to raise TS by 13.5 units; in the same table, caroic acid (applied at 70.degree. C. and pH 10, followed by alkaline extraction) lowers the BL and TS a 0.36 and 8.6 units, respectively. Caroic acid treatment, then, is taught to lower BL and TS from 0.05-0.58 units and 1.4-8.6 units, respectively; while oxygenation/ozonation are taught to lower BL from 0.07-0.53 units and to "lower" TS from 17.7-(-13.5) units. The point of this analysis of the Hammann et al. data is that the Hammann et al. actually tends to Suggest that caroic acid bleaching stages are comparable in their effect on the mechanical properties of pulp to oxygenation and ozonation stages. Of course, the Hammann et al. reference also teaches using caroic acid under rather demanding conditions.
Springer et al., U.S. Pat. No. 4,756,800, teach that pulp can be bleached by monoperoxysulfuric acid salts in an alkaline reaction mixture that comprises cupric ions. The patent teaches that good results are obtained when the pH is maintained at from about 12 to about 12.9.
Ragauskas et al., "Bleaching with Dimethyldioxirane", Bleaching Fundamentals, pp. 29-38, Non-Chlorine Bleaching Conference, Hilton Head, S.C., March 1993, teaches that Oxone may be reacted with acetone to produce dimethyldioxirane, which can be used to bleach pulp. The Ragauskas reference teaches that bleaching with dimethyldioxirane is best performed at a pH of 7, and that the bleaching reaction is most efficient at 80.degree.C.
Meier et al., U.S. Pat. No. 5,091,054, teach that subsequent delignification and bleaching can be enhanced by pretreating lignocellulosic materials such as wood chips, kraft pulp, and the like with peroxymonosulfuric acid or its salts.